Instantaneous frequency diversity radar system

ABSTRACT

1. An apparatus for generating instantaneous frequency diversity signals comprising: MEANS PROVIDING PULSE MODULATED SIGNALS; AND MEANS SAMPLING SAID SIGNALS AT SELECTED INTERVALS FOR PROVIDING OUTPUT SIGNALS OF INSTANTANEOUS FREQUENCY DIVERSITY.

tlnited States atent 91 eans et al.

[ INSTANTANEOUS REQUENCY DIVERSITY RADAR SYSTEM Inventors: Allen F.Beans, lvyland; Alfred M.

Edowes, Doylestown; Richard P.

Gagliardi, Philadelphia; Edward E. Koos, Hatboro; all of Pa.

Primary ExaminerMalcolm F. Hubler Attorney-G. J. Rubens and Henry Hansen[73] Assignee: The United States of America, as EXEMPLARY CLAIMrepresented by the Secretary of the 1. An apparatus for generatinginstantaneous frequen- Navy cy diversity signals comprising: meansproviding pulse modulated signals; and [22] led: July 1966 meanssampling said signals at selected intervals for 21 App| 5 9 010providing output signals of instantaneous frequency diversity.

[52] US. Cl. ..343/l7.1 R 9 Claims, 6 Drawing Figures 12 F wu) 1e P t?19 MAGNETRON MULTIPACTOR 7 2w??? l g: F (w) F (w) 1? 1/ M D L 1: 1MZETJTA'T Q R PROCESSING M XER CIRCUITRY DETECTOR Patented April 24,1973 12? F uu) F(UJ/) L 197 FOR) 20? 22 L BANDPASS MAGNETRON VMULTIPACTOR FILTER DUpLE-XER I F -(w) F (wk 17 -'"""*1 T m l/ MODULATORSAMPLING I MODULATOR BIAS T.W.T. L AMP.

32 3o 26 v 7 7 T F 1 nocsssme 1F CIRCUITRY 3 XER A DETECTOR f MMMMMWMFlg. 2Q

Ta U U Fig. 2b

MFR} 2C g LINE SPACING INVENTORS ALLEN F. BEANS ALFRED M. EDDOWESRICHARD P. GAGLIARDI EDWARD E. KOOS INSTANTANEOUS FREQUENCY DIVERSITYRADAR SYSTEM The invention described herein may be manufactured and usedby or for The Government of the United States of America forgovernmental purposes without the payment of any royalties thereon ortherefor.

The present invention relates to radar systems, and more particularly toan instantaneous frequency diversity pulse radar system for providingimproved target detection capabilities in the presence of clutterconditions.

Radar systems operating in a ground or turbulent sea backgroundenvironment produce electromagnetic returns which suffer not only arandom amplitude variation as a function of the incident frequency, butalso a random phase variation. Both can be attributed to the'randomnature of the scattering cross section of the complex target. Since thescattering cross section of any target is a function of incidentfrequency of wavelength (among other things), illuminating the complextarget with many frequencies simultaneously will tend to decorrelate thereturns from random scatters instantaneously on a single pulse basis.This may be accomplished by processing the return signal through areceiver including a noise-free replica of the transmitted pulse as aninput to a broad band mixer operating in its non-linear region, followedby an integrator to realize the cross-correlation function of theoptimum receiver.

Attempts at synthesis of a broad band frequency spectrum, although notinstantaneous, have been made with one particular type utilizing asingle 'side band generator in a closed loop at X band. This techniqueattempts to linearly superimpose many frequencies related by a leastcommon multiple to cause beat or interference patterns resulting in asin x/x amplitude modulated wave. Due to the finite path length of theloop and associated transit time of the loop, however, only narrowfrequency bands can be accommodated since the proper phase relationshipto cause constructive interference patterns at the right point in timecannot be maintained over wide bands of frequencies. Additionally, thismethod requires extreme frequency and phase stability in all thecomponents which impose a stringent linearity requirement across thefrequency band. Detracting from this method of synthesis is also therequirement for many pieces of complicated test equipment to be used,not only for the initial setup, but also for constant monitoring.

According, the general purpose of the present invention is to overcomethe aforementioned disadvantages by periodically sampling a signal inthe time domain, which as will be described hereinafter, yields periodicrepetition in the frequency domain. The result of this sampling is thecreation of an amplitude modulated carrier wave whose envelope (aftergating) is in the form of bursts of sine x/x time domain pickets bywhich the synthesis of a broadband instantaneous frequency spectrum isaccomplished.

in accordance with one embodiment of the invention,'the carrierfrequency of the radar transmitter is modulated at a relatively highpulse repetition rate and then additionally modulated at a lowrepetition rate,

which may for example, be the system repetition rate. As illustrated inPrinciples of Radar by REINTJES & COATE, 3rd edition, on pages 346through 349, a

pulsed RF signal can be represented by a Fourier series having afrequency spectrum enclosed by a sin x/x envelope. By gating the highfrequency pulsed RF signals at the system repetition rate, the RF pulsesare further amplitude modulated. The frequency spectrum of this signalis then characterized by a plurality of sin x/x waveforms having aspacing equal to the period of the high frequency repetition rate withthe spectral lines occurring at multiples of the low frequencyrepetition rate. The result of this gating or modulation is to producean instantaneous frequency spectrum which in the time domain appears asan amplitude modulated carrier wave whose envelope is in the form ofbursts. Accordingly, by transmitting such a signal, a target isilluminated with many frequencies simultaneously and the returns from acomplex target comprising random scatter will tend to decorrelateinstantaneously on a single pulse basis in the receiver whereas returnsfrom a simple target will correlate and produce an output, afterintegration and detection, which is readily discernible.

In accordance with an additional feature of the present invention, areceiver is utilized which computes the auto correlation of the receivedsignal with itself by employing a mixer operating in the square lawregion thereby giving the product of the signal with itself. Subsequentnarrow banding in the IF and further multiplication in the detector,followed by integration of an A" scope complete the auto correlationmechanization.

An object of the present invention is therefore the synthesis of abroadband frequency spectrum instantaneously by sampling a signal in thetime domain which yields periodic repetition in the frequency domain,the result of which is the creation of an amplitude modulated carrierwave whose envelope is in the form of bursts of sin x/x time pickets.

A further object of the invention is to provide an instantaneousfrequency diversity radar system which instantaneously illuminates atarget with diverse frequencies and in which echo return signals fromcomplex targets tend to decorrelate instantaneously on a single pulsebasis thereby improving the signal to clutter ratio of the system.

Still a further object of the invention is to provide signal to clutterenhancement of target return signals by decorrelating random scattersfrom a complex target by employing auto correlation techniques.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawing in which:

FIG. 1 is a block diagram of a radar system employing frequencydiversity techniques in transmission and auto correlation techniques inreception;

FlGS. 2a-2c illustrate typical amplitude vs. time waveforms of signalsassociated with the embodiment of FIG. 1;

FIG. 3 illustrates a typical frequency spectrum of a transmitted signal;and

FIG. 4 shows a step function of unitary amplitude vs. time.

Referring now to the drawing there is shown in FIG. 1 an embodiment ofthe invention comprising a magnetron 12 being pulsed by a modulator 14by techniques well known to those skilled in the art. For example, themagnetron may be operating at 9.0 Gc with the modulator l4 pulsing themagnetron at a 2 kc. rate with a 2 microsecond pulse width. The outputof the magnetron is illustrated in FIG. 2a as a 2 microsecond burst ofRF energy. This signal is applied to a multipactor 16 which may besimilar to those illustrated in The Microwave Journal, Mar. l962 atpages 93 through 98, or as illustrated in the Apr. 1962 publication ofProceedings of the IRE at pages 442 through 450. The multipactor isessentially a voltage-controlled switch having one input and two outputports, which in the absence of the control voltage, operates as atransmission device to a first output port and a reflecting device tothe second port, whereas in the presence of a control voltage, themultipactor is a reflecting device at the first output port and atransmission device at the second output port. The multipactor iscapable of power levels greater than 2.5 megawatts with minimuminsertion losses and a high degree of isolation to incident power in thereflecting condition.

A sampling modulator 18 is used to provide the control voltage orquenching voltage as it is also referred to, for the multipactor l6 andsince the switching times of the multipactor are less than a nanosecond,the output from the magnetron 12 may be modulated or sampled at veryhigh frequencies so as to produce time pickets. For example, if thesampling modulator 18 produces a modulating pulse as illustrated in FIG.2b, then the non-reflecting output port of the multipactor 16 will be aseries of RF pickets with an envelope similar to that illustrated inFIG. comprising a carrier frequency,f,-, with a pulse spacing determinedby the period of the sampling modulation as illustrated in FIG. 2c. Theother port of the multipactor 16 is terminated in a terminationimpedance 17 which dissipates the interpulse energy from the magnetron12. The nonreflected signal from the multipactor 16 is then appliedthrough a bandpass wave guide filter 19 to a duplexer 20 and then to anantenna 22 for illuminating a target. The filter 19 or spectrumtruncator as is also referred to functions to reduce the spectrum of thetransmitted signal as will be described hereinafter. Although the filteris illustrated as appearing between the multipactor l6 and the duplexer20, it is contemplated that the same result (spectrum limiting) could beachieved by inserting a filter between the sampling modulator l8 and themultipactor 16 as illustrated by the dotted block. In this case, moreconventional bandpass filters could be employed since microwavefrequencies are not involved. The duplexer 20 may be a gas-dischargetube. a phase-shift wave guide, or a ferrite duplexer as described inIntroduction to Radar Systems, by Merrill l. Skolnik, 1962) at pages 395through 403.

To better understand how the aforementioned arrangement of elementsproduces an instantaneous frequency diversity signal, the followinganalytical discussion is presented.

Assume that the output of the magnetron I2 is defined by some function,f,-(t), which may be of a sinusoidal nature such as cos w,1,where w, isthe carrier frequency of the magnetron oscillator. Assume further thatthe modulating signal or gating signal from the modulator I4 is definedas follows:

where U is a step of unitary amplitude between the limits of1- I 0 and 0t 'r, where 21' is the pulse width of the modulator 14 as illustrated inH0. 4.

Since amplitude modulation of a carrier signal is mathematically definedas the product of the carrier signal and the modulating signal, thefollowing expression is obtained:

where F,,,(w) output spectrum from the magnetron, and U dummy variableof integration after integration,

Now, sampling this spectrum at a rate determined by the samplingmodulator 18, the expression for this sampling function in the timedomain is as follows:

where:

f,(t) sampling function in the time domain and T= period of the samplingmodulator Then, transforming this function to the frequency domain,there is obtained:

1"! o E 6(wnw !1==oo Where:

F,(w) output spectrum of the sampling modulator,

and w,,= 21r/T Again, multiplication in the time domain is convolutionin the frequency domain times /21r, therefore the output spectrum fromthe multipactor 16 is:

au ul M) I Sm s a m z. N) U I v n V 2w zl altw s) w t/ where:

A utpul spectrum from the multipactor, and

5 dummy variable of integration then, after integration:

which defines an instantaneous frequency spectrum of infinite bandwidth.Since, however, it is desired to operate only over a particular band,the broadband wave guide filter 19 (well known to those skilled in theart) centered about m, can be inserted between the multipactor l6 andthe duplexer 20 for bandwidth limiting. If the filter 19 has a bandpassbetween selectively variable frequencies of k and k then theinstantaneoustransmitted spectrum at the antenna is:

where:

F,,(w) instantaneous output spectrum of the transmitted signal,bandwidth limited.

Referring now to FIG. 3, there is illustrated a frequency spectrum ofthe transmitted signal, F,,(m) with the horizontal axis being defined asw and the vertical axis indicating the relative amplitudes of theFourier components. As illustrated, each sin x/x function is spaced fromeach other by an amount equal to 21r/t and the width of the functionbetween the first zero crossings is equal to 41r/a. The number ofspectral lines is a function of the pulse repetition rate of the radarsystem. Accordingly, the line spacing is equal to 21r/T with theamplitude of each line representing the relative amplitude of theparticular harmonic.

Referring again to FIG. 1, echo returns are received through the antenna22 and passed through the duplexer to a traveling wave tube amplifier 24where the echo returns are amplified and applied to a mixer 26 having asquare law characteristic; that is, a signal applied at its input ismultiplied by itself. This type conversion is referred to as autocorrelation. To insure that the mixer is operating in the square lawregion, a bias voltage is provided from a bias supply 28. The output ofthe mixer 26 is a series of video pulses spaced from each other by anamount equal to the sampling rate of the transmitted signal. These videopulses are then applied to an IF amplifier and detector circuit 30having an IF bandpass centered about the frequency of the returnsignals; that is, the frequency of the IF amplifier is the same as therepetition rate of the sampling modulator 18. If the sampling modulatoris operating at a 5 megacycle rate, the return signals will also be at a5 megacycle rate; accordingly, the IF amplifier will have a centerfrequency at 5 megacycles and a bandwidth sufficient to provide pulse topulse integration. The output of the amplifier is then detected andapplied to radar processing circuitry 32 from which target rangeinformation can be obtained.

In operation then, the magnetron 12 is pulsed at a particular systemrepetition rate, which for purposes of illustration may be for 2microsecond intervals at a 2 kc. rate. The output of the magnetron isthen applied to the multipactor 16 which samples the pulses RF signal ata rate determined by the sampling modulator 18. For example, if thesampling modulator is operating at a 5.0 megacycle repetition rate, theoutput of the multipactor 16 is then a series of pulse pickets having apulse width equal to that of the sampling signal for a 2 microsecondduration. As a result of the carrier frequency gating and subsequentsampling by the multipactor, an amplitude modulated carrier frequency isproduced which in the frequency domain represents an instantaneousfrequency diversity signal.

Whereas echo signals from a complex target illuminated by theaforementioned signal have no common correlation (since the scatteringcross section per unit area increases with increasing frequency),returns from a simple target will correlate and produce an output whichwill be readily discernible after integration and detection.

Consider, for example, that it is desired to locate a simple target in acomplex environment. in such an environment, typical pulse radar systemsemploying either auto correlation or cross correlation techniques wouldreceive large clutter return signals (produced by the wave motion) andhence the signal to clutter ratio would be very poor. Accordingly, theprobability of detection would be considerably reduced. By illuminatingthe target with many frequencies simultaneously, however, the returnsfrom random scatters will decorrelate instantaneously on a single pulsebasis thereby improving the signal to clutter ratio and hence theprobability of detection.

Accordingly, there is illustrated herein a technique for providinginstantaneous frequency diversity signals for transmission to a targetwhich in the presence of a clutter environment will provide signal toclutter enhancement in the received signal thereby increasing theprobability of detection.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

l. An apparatus for generating instantaneous frequency diversity signalscomprising:

means providing pulse modulated signals; and

means sampling said signals at selected intervals for providing outputsignals of instantaneous frequency diversity.

2. An apparatus as recited in claim 1 further comprising:

means radiating said output signals to an object and receivingreflections therefrom; and

means correlating said reflections and providing an output from whichrange information is obtained.

3. An apparatus as recited in claim 2 wherein said means sampling saidsignal comprises:

sampling modulator means providing a control voltage; and

a multipactor receiving said control voltage and passing said modulatedsignals only in the presence of said control voltage, whereby the outputof said multipactor comprises pulse pickets having an instantaneousfrequency diversity.

4. An apparatus as recited in claim 3 wherein said means sampling saidsignal further comprises:

filter means connected to the output of said multipactor for limitingthe bandwidth of said instantaneous frequency diversity signals.

5. An apparatus as recited in claim 3 wherein said sampling modulatormeans comprises:

7 8 a sampling modulator; and means integrating the output signalscomprises: filter means connected to the output of said sampling anintermediate frequency amplifier having a bandmodulator means forlimiting the bandwidth of pass characteristic centered about therepetition Said control frequency of said sampling means for integrating6. An apparatus as recited in claim 3 wherein said the output signalsfrom said mixer;

means correlating said reflections comprises:

a mixer operating in a square law region for yielding the product of thereceived reflections with itself thereby providing a maximum value of anautocorrelation function; 10

means integrating the output signals from said mixer for providing anoutput from which range information is obtained.

7. An apparatus as recited in claim 3 wherein said means providing apulse modulated signal comprises: IS

a magnetron;

means modulating said magnetron for providing said pulse modulatedsignals.

8. An apparatus as recited in claim 6 wherein said detector meansreceiving the integrated signal and providing a detected output havingan amplitude proportional to the integrated signal from said amplifier;and

means processing said detected signal for determining object rangeinformation.

9. An apparatus as recited in claim 6 wherein said means radiating saidoutput signals comprises:

a duplexer receiving said instantaneous frequency diversity signals; and

an antenna coupled to said duplexer for radiating said signals.

1. An apparatus for generating instantaneous frequency diversity signalscomprising: means providing pulse modulated signals; and means samplingsaid signals at selected intervals for providing output signals ofinstantaneous frequency diversity.
 2. An apparatus as recited in claim 1further comprising: means radiating said output signals to an object andreceiving reflections therefrom; and means correlating said reflectionsand providing an output from which range information is obtained.
 3. Anapparatus as recited in claim 2 wherein said means sampling said signalcomprises: sampling modulator means providing a control voltage; and amultipactor receiving said control voltage and passing said modulatedsignals only in the presence of said control voltage, whereby the outputof said multipactor comprises pulse pickets having an instantaneousfrequency diversity.
 4. An apparatus as recited in claim 3 wherein saidmeans sampling said signal further comprises: filter means connected tothe output of said multipactor for limiting the bandwidth of saidinstantaneous frequency diversity signals.
 5. An apparatus as recited inclaim 3 wherein said samPling modulator means comprises: a samplingmodulator; and filter means connected to the output of said samplingmodulator means for limiting the bandwidth of said control voltage. 6.An apparatus as recited in claim 3 wherein said means correlating saidreflections comprises: a mixer operating in a square law region foryielding the product of the received reflections with itself therebyproviding a maximum value of an autocorrelation function; meansintegrating the output signals from said mixer for providing an outputfrom which range information is obtained.
 7. An apparatus as recited inclaim 3 wherein said means providing a pulse modulated signal comprises:a magnetron; means modulating said magnetron for providing said pulsemodulated signals.
 8. An apparatus as recited in claim 6 wherein saidmeans integrating the output signals comprises: an intermediatefrequency amplifier having a bandpass characteristic centered about therepetition frequency of said sampling means for integrating the outputsignals from said mixer; detector means receiving the integrated signaland providing a detected output having an amplitude proportional to theintegrated signal from said amplifier; and means processing saiddetected signal for determining object range information.
 9. Anapparatus as recited in claim 6 wherein said means radiating said outputsignals comprises: a duplexer receiving said instantaneous frequencydiversity signals; and an antenna coupled to said duplexer for radiatingsaid signals.